In systems for sensing a physical parameter, a capacitive sensing element is commonly used wherein a plate or diaphragm moves in response to a force, pressure or temperature change. The change of plate position varies the width of a dielectric gap, changing the capacitance of the sensing element. The variation in dielectric gap is a function of the physical parameter which is fully determined by the mechanics of the device, and may be readily modeled or empirically measured. Moreover, the variation in capacitance may be a linear function of the change in gap, so that by measuring a change in capacitance of the sensor one effectively measures the desired physical parameter.
In practice, the change in capacitance is measured by incorporating the sensor as an active element of a circuit, and measuring the change in frequency, phase or magnitude of a signal applied to or formed by the circuit. Applicant's earlier U.S. Pat. No. 4,054,833 shows circuitry of this type, wherein the measurement circuitry employs a reference capacitor element in series with a capacitive sensing element. A switching circuit cyclically applies different voltages to the reference and to the sensing elements, and a feedback circuit adjusts one voltage so that the signal at the junction of the two elements is maintained substantially at a fixed value. The value of the feedback voltage necessary to achieve a stationary signal in the circuit is representative of the capacitance value. Applicant's aforesaid U.S. patent shows a circuit wherein the feedback signal is a linear function of sensor plate displacement.
In prior art circuits, it is advantageous to employ as a sensor a central diaphragm type sensor with the two opposed sides forming the basic capacitance measurement network. In such a sensor, a change in gap of one side caused by motion of the diaphragm produces an equal and oppositely-directed change in gap of the other side. In that case the reference and source capacitance each vary. However, since to a first approximation the changes in the two capacitances are substantially equal and opposite, the net variation is greater than for a single-sided sensor. Thus, when such a push-pull sensing network is employed, the symmetry of the network results in a sensitive determination of the position of the central diaphragm for small displacements.
In practice, a variable capacitance sensing element is a physical structure of plates, housing conductive leads, spacers and the like which have a certain amount of leakage. This leakage capacitance introduces non-linearities into the measurement network. Other non-linearities are also presented, due to non-parallelism of the plates, departures from plate flatness, change in diaphragm stiffness at larger displacements, and other physical factors. In the case of a push-pull capacitive sensor, each side of the sensor contributes its own parasitic or leakage capacitance, in addition to the other non-linearities and variations due to diaphragm displacement. For larger displacements, the added leakage capacitances and non-linearities limit the range of the parameter that can be effectively measured by the sensor circuitry.
Therefore it is desirable to correct the effects of sensor capacitance non-linearity and to extend the range of accuracy of a sensing circuit.